Warn for using ? in executable code but outside NonNullTypes(true) context (#28842)

src/Compilers/CSharp/Portable/Binder/Binder.cs

@@ -329,6 +329,9 @@ internal virtual Imports GetImports(ConsList<Symbol> basesBeingResolved)
             return _next.GetImports(basesBeingResolved);
         }
 
+        protected virtual bool InExecutableBinder
+            => _next.InExecutableBinder;
+
         /// <summary>
         /// The type containing the binding context
         /// </summary>

src/Compilers/CSharp/Portable/Binder/Binder_Symbols.cs

@@ -338,11 +338,23 @@ internal NamespaceOrTypeOrAliasSymbolWithAnnotations BindNamespaceOrTypeOrAliasS
             {
                 case SyntaxKind.NullableType:
                     {
-                        TypeSyntax typeArgumentSyntax = ((NullableTypeSyntax)syntax).ElementType;
+                        var nullableSyntax = (NullableTypeSyntax)syntax;
+                        TypeSyntax typeArgumentSyntax = nullableSyntax.ElementType;
                         TypeSymbolWithAnnotations typeArgument = BindType(typeArgumentSyntax, diagnostics, basesBeingResolved);
                         TypeSymbolWithAnnotations constructedType;
                         if (Compilation.IsFeatureEnabled(MessageID.IDS_FeatureStaticNullChecking))
                         {
+                            if (InExecutableBinder)
+                            {
+                                // Inside a method body or other executable code, we can afford to pull on NonNullTypes symbol or question IsValueType without causing cycles.
+                                // Types created outside executable context should be checked by the responsible symbol (the method symbol checks its return type, for instance).
+
+                                if (!typeArgument.IsValueType && NonNullTypesContext.NonNullTypes != true)
+                                {
+                                    diagnostics.Add(ErrorCode.WRN_MissingNonNullTypesContextForAnnotation, nullableSyntax.QuestionToken.GetLocation());
+                                }
+                            }
+
                             constructedType = typeArgument.SetIsAnnotated(Compilation);
                             if (!ShouldCheckConstraints)
                             {

src/Compilers/CSharp/Portable/Binder/BuckStopsHereBinder.cs

@@ -56,6 +56,8 @@ protected override LocalFunctionSymbol LookupLocalFunction(SyntaxToken nameToken
 
         internal override uint LocalScopeDepth => Binder.ExternalScope;
 
+        protected override bool InExecutableBinder => false;
+
         internal override bool IsAccessibleHelper(Symbol symbol, TypeSymbol accessThroughType, out bool failedThroughTypeCheck, ref HashSet<DiagnosticInfo> useSiteDiagnostics, ConsList<Symbol> basesBeingResolved)
         {
             failedThroughTypeCheck = false;

src/Compilers/CSharp/Portable/Binder/ExecutableCodeBinder.cs

@@ -46,6 +46,9 @@ internal override Symbol ContainingMemberOrLambda
             get { return _memberSymbol ?? Next.ContainingMemberOrLambda; }
         }
 
+        protected override bool InExecutableBinder
+            => true;
+
         internal Symbol MemberSymbol { get { return _memberSymbol; } }
 
         internal override Binder GetBinder(SyntaxNode node)

src/Compilers/CSharp/Portable/Binder/InMethodBinder.cs

@@ -66,6 +66,8 @@ protected override LocalFunctionSymbol LookupLocalFunction(SyntaxToken nameToken
 
         internal override uint LocalScopeDepth => Binder.TopLevelScope;
 
+        protected override bool InExecutableBinder => true;
+
         internal override Symbol ContainingMemberOrLambda
         {
             get

src/Compilers/CSharp/Portable/Binder/WithMethodTypeParametersBinder.cs

@@ -20,6 +20,8 @@ internal WithMethodTypeParametersBinder(MethodSymbol methodSymbol, Binder next)
             _methodSymbol = methodSymbol;
         }
 
+        protected override bool InExecutableBinder => false;
+
         internal override Symbol ContainingMemberOrLambda
         {
             get

src/Compilers/CSharp/Portable/CSharpResources.Designer.cs

@@ -13970,7 +13970,7 @@ internal class CSharpResources {
         }
         
         /// <summary>
-        ///   Looks up a localized string similar to The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context..
+        ///   Looks up a localized string similar to The suppression operator (!) should be used in code with a &apos;[NonNullTypes(true/false)]&apos; context..
         /// </summary>
         internal static string WRN_MissingNonNullTypesContext {
             get {
@@ -13979,7 +13979,7 @@ internal class CSharpResources {
         }
         
         /// <summary>
-        ///   Looks up a localized string similar to The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context..
+        ///   Looks up a localized string similar to The suppression operator (!) should be used in code with a &apos;[NonNullTypes(true/false)]&apos; context..
         /// </summary>
         internal static string WRN_MissingNonNullTypesContext_Title {
             get {
@@ -13987,6 +13987,24 @@ internal class CSharpResources {
             }
         }
         
+        /// <summary>
+        ///   Looks up a localized string similar to The annotation for nullable reference types should only be used in code within a &apos;[NonNullTypes(true)]&apos; context..
+        /// </summary>
+        internal static string WRN_MissingNonNullTypesContextForAnnotation {
+            get {
+                return ResourceManager.GetString("WRN_MissingNonNullTypesContextForAnnotation", resourceCulture);
+            }
+        }
+        
+        /// <summary>
+        ///   Looks up a localized string similar to The annotation for nullable reference types should only be used in code within a &apos;[NonNullTypes(true)]&apos; context..
+        /// </summary>
+        internal static string WRN_MissingNonNullTypesContextForAnnotation_Title {
+            get {
+                return ResourceManager.GetString("WRN_MissingNonNullTypesContextForAnnotation_Title", resourceCulture);
+            }
+        }
+        
         /// <summary>
         ///   Looks up a localized string similar to Parameter &apos;{0}&apos; has no matching param tag in the XML comment for &apos;{1}&apos; (but other parameters do).
         /// </summary>

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -5369,6 +5369,12 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="WRN_NullabilityMismatchInTypeParameterConstraint_Title" xml:space="preserve">
     <value>The type cannot be used as type parameter in the generic type or method. Nullability of type argument doesn't match constraint type.</value>
   </data>
+  <data name="WRN_MissingNonNullTypesContextForAnnotation" xml:space="preserve">
+    <value>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</value>
+  </data>
+  <data name="WRN_MissingNonNullTypesContextForAnnotation_Title" xml:space="preserve">
+    <value>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</value>
+  </data>
   <data name="ERR_ExplicitNullableAttribute" xml:space="preserve">
     <value>Explicit application of 'System.Runtime.CompilerServices.NullableAttribute' is not allowed.</value>
   </data>
@@ -5382,10 +5388,10 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
     <value>Please use language version {0} or greater to use the NonNullTypes attribute.</value>
   </data>
   <data name="WRN_MissingNonNullTypesContext" xml:space="preserve">
-    <value>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</value>
+    <value>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</value>
   </data>
   <data name="WRN_MissingNonNullTypesContext_Title" xml:space="preserve">
-    <value>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</value>
+    <value>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</value>
   </data>
   <data name="ERR_NonTaskMainCantBeAsync" xml:space="preserve">
     <value>A void or int returning entry point cannot be async</value>
@@ -5522,4 +5528,4 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_OutVariableCannotBeByRef" xml:space="preserve">
     <value>An out variable cannot be declared as a ref local</value>
   </data>
-</root>
\ No newline at end of file
+</root>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -1616,6 +1616,7 @@ internal enum ErrorCode
         WRN_MissingNonNullTypesContext = 8629,
         ERR_NonNullTypesNotAvailable = 8630,
         WRN_NullabilityMismatchInTypeParameterConstraint = 8631,
+        WRN_MissingNonNullTypesContextForAnnotation = 8632,
     }
     // Note: you will need to re-generate compiler code after adding warnings (build\scripts\generate-compiler-code.cmd)
 }

src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs

@@ -348,6 +348,7 @@ internal static int GetWarningLevel(ErrorCode code)
                 case ErrorCode.WRN_TupleBinopLiteralNameMismatch:
                 case ErrorCode.WRN_TypeParameterSameAsOuterMethodTypeParameter:
                 case ErrorCode.WRN_MissingNonNullTypesContext:
+                case ErrorCode.WRN_MissingNonNullTypesContextForAnnotation:
                     return 1;
                 default:
                     return 0;

src/Compilers/CSharp/Portable/Generated/ErrorFacts.Generated.cs

@@ -204,6 +204,7 @@ public static bool IsWarning(ErrorCode code)
                 case ErrorCode.WRN_NoBestNullabilityConditionalExpression:
                 case ErrorCode.WRN_MissingNonNullTypesContext:
                 case ErrorCode.WRN_NullabilityMismatchInTypeParameterConstraint:
+                case ErrorCode.WRN_MissingNonNullTypesContextForAnnotation:
                     return true;
                 default:
                     return false;

src/Compilers/CSharp/Portable/Symbols/MethodSymbol.cs

@@ -964,7 +964,7 @@ internal virtual bool SynthesizesLoweredBoundBody
         {
             get
             {
-                return (AssociatedSymbol ?? ContainingType)?.NonNullTypes;
+                return (AssociatedSymbol ?? ContainingSymbol)?.NonNullTypes;
             }
         }
 

src/Compilers/CSharp/Portable/xlf/CSharpResources.cs.xlf

@@ -403,13 +403,23 @@
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation_Title">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext_Title">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_TypeParameterSameAsOuterMethodTypeParameter">

src/Compilers/CSharp/Portable/xlf/CSharpResources.de.xlf

@@ -403,13 +403,23 @@
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation_Title">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext_Title">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_TypeParameterSameAsOuterMethodTypeParameter">

src/Compilers/CSharp/Portable/xlf/CSharpResources.es.xlf

@@ -403,13 +403,23 @@
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation_Title">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext_Title">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_TypeParameterSameAsOuterMethodTypeParameter">

src/Compilers/CSharp/Portable/xlf/CSharpResources.fr.xlf

@@ -403,13 +403,23 @@
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation_Title">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext_Title">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_TypeParameterSameAsOuterMethodTypeParameter">

src/Compilers/CSharp/Portable/xlf/CSharpResources.it.xlf

@@ -403,13 +403,23 @@
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation_Title">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext_Title">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_TypeParameterSameAsOuterMethodTypeParameter">

src/Compilers/CSharp/Portable/xlf/CSharpResources.ja.xlf

@@ -403,13 +403,23 @@
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation_Title">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext_Title">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_TypeParameterSameAsOuterMethodTypeParameter">

src/Compilers/CSharp/Portable/xlf/CSharpResources.ko.xlf

@@ -403,13 +403,23 @@
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation_Title">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext_Title">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_TypeParameterSameAsOuterMethodTypeParameter">

src/Compilers/CSharp/Portable/xlf/CSharpResources.pl.xlf

@@ -403,13 +403,23 @@
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation_Title">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext_Title">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_TypeParameterSameAsOuterMethodTypeParameter">

src/Compilers/CSharp/Portable/xlf/CSharpResources.pt-BR.xlf

@@ -403,13 +403,23 @@
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation_Title">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext_Title">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_TypeParameterSameAsOuterMethodTypeParameter">

src/Compilers/CSharp/Portable/xlf/CSharpResources.ru.xlf

@@ -403,13 +403,23 @@
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation_Title">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext_Title">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_TypeParameterSameAsOuterMethodTypeParameter">

src/Compilers/CSharp/Portable/xlf/CSharpResources.tr.xlf

@@ -403,13 +403,23 @@
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation_Title">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext_Title">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_TypeParameterSameAsOuterMethodTypeParameter">

src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hans.xlf

@@ -403,13 +403,23 @@
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation_Title">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext_Title">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_TypeParameterSameAsOuterMethodTypeParameter">

src/Compilers/CSharp/Portable/xlf/CSharpResources.zh-Hant.xlf

@@ -403,13 +403,23 @@
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_MissingNonNullTypesContextForAnnotation_Title">
+        <source>The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</source>
+        <target state="new">The annotation for nullable reference types should only be used in code within a '[NonNullTypes(true)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_MissingNonNullTypesContext_Title">
-        <source>The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</source>
-        <target state="new">The suppression operator (!) should be used in code with a `[NonNullTypes(true/false)]` context.</target>
+        <source>The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</source>
+        <target state="new">The suppression operator (!) should be used in code with a '[NonNullTypes(true/false)]' context.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_TypeParameterSameAsOuterMethodTypeParameter">

src/Compilers/CSharp/Test/Emit/Attributes/AttributeTests_Nullable.cs

@@ -579,7 +579,7 @@ static void Main()
         new C<B2?>();
     }
 }";
-            var comp2 = CreateCompilation(source2, parseOptions: TestOptions.Regular8, references: new[] { comp.EmitToImageReference() });
+            var comp2 = CreateCompilation(new[] { source2, NonNullTypesTrue, NonNullTypesAttributesDefinition }, parseOptions: TestOptions.Regular8, references: new[] { comp.EmitToImageReference() });
             comp2.VerifyDiagnostics();
 
             var type = comp2.GetMember<NamedTypeSymbol>("C");
@@ -681,10 +681,10 @@ static void Main()
         new C<object, string>();
     }
 }";
-            var comp2 = CreateCompilation(new[] { source, source2 }, parseOptions: TestOptions.Regular8);
+            var comp2 = CreateCompilation(new[] { source, source2, NonNullTypesTrue, NonNullTypesAttributesDefinition }, parseOptions: TestOptions.Regular8);
             comp2.VerifyEmitDiagnostics();
 
-            comp2 = CreateCompilation(source2, parseOptions: TestOptions.Regular8, references: new[] { comp.EmitToImageReference() });
+            comp2 = CreateCompilation(new[] { source2, NonNullTypesTrue, NonNullTypesAttributesDefinition }, parseOptions: TestOptions.Regular8, references: new[] { comp.EmitToImageReference() });
             comp2.VerifyEmitDiagnostics();
 
             var type = comp2.GetMember<NamedTypeSymbol>("C");
@@ -1292,7 +1292,7 @@ static void G()
         F((object? o) => { });
     }
 }";
-            var comp = CreateCompilation(source, parseOptions: TestOptions.Regular8, options: TestOptions.ReleaseModule);
+            var comp = CreateCompilation(new[] { source, NonNullTypesTrue, NonNullTypesAttributesDefinition }, parseOptions: TestOptions.Regular8, options: TestOptions.ReleaseModule);
             comp.VerifyEmitDiagnostics(
                 // (9,12): error CS0518: Predefined type 'System.Runtime.CompilerServices.NullableAttribute' is not defined or imported
                 //         F((object? o) => { });
@@ -1311,7 +1311,7 @@ static void M()
         L();
     }
 }";
-            var comp = CreateCompilation(source, parseOptions: TestOptions.Regular8, options: TestOptions.ReleaseModule);
+            var comp = CreateCompilation(new[] { source, NonNullTypesTrue, NonNullTypesAttributesDefinition }, parseOptions: TestOptions.Regular8, options: TestOptions.ReleaseModule);
             comp.VerifyEmitDiagnostics(
                 // (5,9): error CS0518: Predefined type 'System.Runtime.CompilerServices.NullableAttribute' is not defined or imported
                 //         object?[] L() => throw new System.NotImplementedException();
@@ -1330,7 +1330,7 @@ static void M()
         L(null, 2);
     }
 }";
-            var comp = CreateCompilation(source, parseOptions: TestOptions.Regular8, options: TestOptions.ReleaseModule);
+            var comp = CreateCompilation(new[] { source, NonNullTypesTrue, NonNullTypesAttributesDefinition }, parseOptions: TestOptions.Regular8, options: TestOptions.ReleaseModule);
             comp.VerifyEmitDiagnostics(
                 // (5,16): error CS0518: Predefined type 'System.Runtime.CompilerServices.NullableAttribute' is not defined or imported
                 //         void L(object? x, object y) { }

src/Compilers/CSharp/Test/Semantic/Semantics/UninitializedNonNullableFieldTests.cs

@@ -385,7 +385,7 @@ internal C(object o)
         P2 = new object?[] { o };
     }
 }";
-            var comp = CreateCompilation(source, parseOptions: TestOptions.Regular8);
+            var comp = CreateCompilation(new[] { source, NonNullTypesTrue, NonNullTypesAttributesDefinition }, parseOptions: TestOptions.Regular8);
             comp.VerifyDiagnostics();
         }
 

src/Compilers/CSharp/Test/Syntax/Diagnostics/DiagnosticTest.cs

@@ -281,6 +281,7 @@ public void WarningLevel_2()
                         case ErrorCode.WRN_NoBestNullabilityConditionalExpression:
                         case ErrorCode.WRN_MissingNonNullTypesContext:
                         case ErrorCode.WRN_NullabilityMismatchInTypeParameterConstraint:
+                        case ErrorCode.WRN_MissingNonNullTypesContextForAnnotation:
                             Assert.Equal(1, ErrorFacts.GetWarningLevel(errorCode));
                             break;
                         case ErrorCode.WRN_InvalidVersionFormat: